Both the automotive industry and law enforcement personnel sometimes encounter problems when vehicles have windows that are coated with films that do not meet state or federal enacted laws. The applied film can change the visible light transmission and reflection characteristics of the entire window assembly. Often, the law enforcement personnel are put at a disadvantage when approaching such vehicle since the personnel cannot readily see into the vehicle.
Currently, there are portable products available on the market which perform at 550 nm and which are used to predict the transmission and reflection performance of the entire visible light spectrum. Such products use a single LED and a single sensor.
In fact, a window film manufacturer might specify the products' entire visible light transmission percentages using this method of prediction. For example, one manufacturer makes dyed films, such as an automotive window film, LLumar® with HPR™ adhesive which specifies the following:
AT 35 GNType: Non-reflectiveSR HPR% Visible Light Transmission (at 550 nm)35% Total Solar Energy Rejection35% Visible Light Reflection7% Ultra-Violet Light Rejection99
However, accurate visible light transmission and reflection measurements require that the measurement method be normalized for every wavelength being measured.
The generally accepted measurement range for visible light is 380-720 nm. Law enforcement personnel cannot receive accurate information from a single wavelength or frequency measurement at 550 nm. The light source from such products does not provide energy across the entire visible light spectrum. Further, the light sensor in such products does not have a “flat” or “the same” response for every wavelength across the spectrum. Similarly, there is currently no known single light source available which provides an ideal light spectrum having equal energy at all wavelengths of light from 380-720 nm.
Visible light transmission (VLT) and reflection meters on the market for the automotive industry use the technique described above. In order to use such products, the law enforcement officer slides the product over an edge of the glass. Prior art FIG. 1 schematically illustrates a transmission meter 2 with a single light emitting unit 3 and a single light receiving unit 5, and an opening 4 between the single light emitting unit 3 and the light receiving unit 5 for receiving a window 7 having a film 8 thereon.
Alternatively, the product can include a separate transmitting unit and a separate receiving unit which are placed on opposite sides of the glass. See for example, U.S. Pat. No. Des. 362,810.
These types of products use a single light source and a single light sensor to estimate the entire visible light transmission (VLT) and reflection characteristics of the glass. FIG. 2 illustrates: the spectrum for a green LED at about 550 nm; the photodiode spectral responsivity curve from 380-720 nm (which is similar to that shown in FIG. 6 herein); and, the output response from the non-linear sensor and the limited Green LED spectrum. Thus, the result is not linear and may not accurately measure the transmission since the green LED is only capable of being detected at about the 550 nm range by the receiving unit.
U.S. Pat. No. 3,910,701 uses alternating or sequential LED sources which then are summed together after the measurement is performed.
An alternate form of measuring light transmission is to use a laboratory instrument such as a spectrometer. A spectrometer uses a single broadband light source and characterizes the received light energy into small light frequency ranges of a few nanometers or less. This laboratory instrument analyzes the received light energy on each pixel or element (normally 2048 or greater) on a CCD (charged coupled device) and normalizes the response for each sensing element.
Mathematical manipulation allows the user to select certain light frequency ranges so that individual wavelength measurements can be made on each uniquely defined light sensing pixel or element, and the resulting summation of the data can be made in the laboratory. Thus, the spectrometer uses a prism to separate a broad-spectrum light source into discrete light frequencies that can be mated with individual sensing elements.